We're limiting to GMail (including google app accounts) so we can focus on nailing the interface and auto-categorization of email. We plan to add support for other providers and possibly self-hosted in the future.
I would include this information on your How Glider Works page, as well as a "Stay Notified" option for visitors who don't use GMail but are interested in Glider. Making this addition will allow you to build targeted lists of potential users for future versions of Glider.
